Many of the negative feedbacks given and received can be avoided by asking the right questions before bidding.  Almost all the 'buyer's regret' can be avoided by asking the right questions.  But what, might you ask, ARE the right questions?  Overall, there are general questions you should ask before considering a bid.  Never assume anything. 
GENERAL QUESTIONS
1.  Is the item in perfect condition, even if it states that it is new?  And what is the buyer's recourse if it arrives in much different condition than stated?  Can it be returned?
2.  If the item is used, is it in working/servicable condition?  And if there are condition issues, how really bad are they?
3.  What are the shipping costs, and does that include insurance?
4.  If return policy is not stated, ask what it is.  For instance, ask if the color/size of the garment is not right for you, are returns accepted? Remember, your attitude as a buyer is as important as the seller's!  If you bid on the purple sweater but it's the wrong purple, you can't  expect to return it if you haven't cleared it with the seller first.
5.  What are the exact dimensions/measurements/size of the item?  You can't expect 2 skeins of 50gm yarn to be the same yardage if they are different types, or even if they are the same maker.  Ask what the yardage is, ask what the waist measurement is, ask how long the pants are, before you bid.
6.  If an item is a name brand, I always ask the sellerr if it is GENUINE.  Many copies are sold as 'authentic' and I've bought them.  And they often turn out not to be the 'real deal'.  So I always ask if an item is genuinely the maker represented in the auction.  If the seller won't say it is the genuine article, I shop elsewhere.
7.  If an item is genuine, will the seller include the original sales receipt?  Ask yourself WHY the seller would only be willing to send you a copy of the original sales slip.  They can (and should) black out their name and address, but if you are buying a genuine designer something, and the seller states they have the original sales slip, the new owner should have it.
8.  In the case of artwork, can the seller provide provenance(the artwork's history of owners, repairs, etc)?  Some sellers sell copies and claim they bought it at a yard/estate/garage sale.  Ask if they have had it evaluated as to its true nature---either original or copy--and bid accordingly. 
9.  If you see something out of line in an auction, and I am not refering to something outside EBAY's protocol.  I am referring to an exorbitant shipping charge, for instance, or an auction that states a pair of vases are being sold, but also states only one is being sold, you need to clarify before bidding.  I once asked about a shipping charge of $50 on an item that should have been around $7 to ship.  The seller agreed, and said she had only been guessing on her shipping cost.  Another time, I wanted to bid on a 'lot' of antique charms, but the auction text stated that the auction was for the bracelet only.  I asked the seller what was for sale, and it turned out that she had made a mistake in the listing.   So ask!

Those are my general questions that I use to base my decision on whether or not to bid.  As you look through auctions, you will begin to have internal questions that will come to mind.  What's been left out?  Do I need more information?  Why are the pictures so blurry?  Can the seller send me clear pictures?  Remember, please be nice, humor never hurts, and be courteous.  I have met some amazing people all over the world via EBAY.  Sometimes, things go wrong, and it's neither the buyer's or seller's fault.  A bit of humor and willingness to work out difficulties inevitably solves the problem.  I am both a buyer and seller, and it is imperative for me to have/give as much information as possible before I buy or sell.  If you ask questions pertinent to your intended buy BEFORE you bid, you can avoid flame wars, negative feedback, and disappointment in your EBAY adventures.